Page 41 - MODELING OF ASPHALT CONCRETE

Modeling of Asphalt Binder Rheology and Its Application to Modified Binders 19

changing to non-Newtonian gradually as the shear rate or stress level increases
(Puzinauskas 1967). Therefore, the relation is not linear and the value of C will
depend on which range is referred to.
2. The parameter, being a measure of non-Newtonian behavior, will naturally
depend on the stress history. Majidzadeh and Schweyer (1965) conducted an
experiment where the C value was measured for the same asphalt under the
same conditions, but the sequence of loading applications was changed. They
compared C measured when the load increments where added in a decreasing
i
sequence of load values with “C “ measured with an increasing sequence of
d
load step values.
3. In addition to the above problems with the C measure another point may be
added here. Constructing the stress-shear rate relation is subjected to the same
controversy associated with apparent viscosity. From a standard creep curve
measured for a non-Newtonian material, the shear rate may never come to an
equilibrium within reasonable time or strain level, therefore, the value of the
shear rate used to construct the stress-shear rate curve will depend on the time
of loading at which this rate is determined. In other words the C value is time
of loading dependent.

The Shear Index Apparent viscosity is plotted versus shear rate on a log-log scale. The
slope of the relation between two different shear rates is determined and considered as
the shear susceptibility parameter. Although early works tried to approximate the
relation as linear (Jimenez and Gallaway 1961; Gallaway 1959), a large number of more
recent studies showed clearly that this is not true. Apparent viscosity is constant up to
the Newtonian behavior limit after which the viscosity starts decreasing continuously
with increasing shear rate.
For asphalt aging studies, the shear index has been used by many researchers. Zube
and Skog (1969), Culley (1969), and Kandhal et al. (1973) all compared viscosity-shear
rate plots of unaged with lab or field aged asphalts. However, since the plots are
nonlinear and temperature dependent researchers selected different ranges of shear
rates and different temperatures. All agreed that an increase in the shear susceptibility
is observed upon aging.
As with other susceptibility parameters, the shear index may be considered as an
arbitrary value. The exact value depends, to a great extent, on the range of shear rate
considered and the temperature at which it is measured. A single shear index may only
indicate the type of behavior expected for an asphalt at the temperature and shear rate
range it is measured at; no simple extrapolation or even interpolation can be made.
Moreover, it still uses the apparent viscosity which is time of loading dependent and if
this is not constant then the plot is very difficult to interpret.
In summary, shear susceptibility parameters are not much better than temperature
susceptibility parameters, or even the single measurements. They are also arbitrary in
nature and do not show high promise for use in asphalt rheological characterization.

Linear Viscoelastic Properties
Although using rheological concepts to characterize asphalts dates back more than 50
years, the cost and availability of equipment hampered the spread of using viscoelasticity
to qualify asphalts and study effect of modifiers. This shortcoming was overcome